Synopsis: Inmate No. 666

Lee Shi Woo, the heir to Seojin Group, is sent to prison for protection—but instead finds himself targeted, as if marked for death by his own family. When a mysterious inmate, Nam Jiho—known by the number 666—saves him, Shi Woo decides to stay close for survival. But the deeper he gets, the clearer it becomes: Jiho might be the most dangerous threat of all.